1. Field of the Invention
The present invention relates to a process that uses an extrusion mold to manufacture thermoplastic resin sheets having along the widthwise direction a first (clear) resin section comprising a first thermoplastic resin and a colored section in which a colored band made of the colored thermoplastic resin is encapsulated within the first resin layer formed with the first thermoplastic resin, and an apparatus for manufacturing such sheets.
The windshield glass used in automobiles, and airplanes, and the laminated glass used in windows of buildings, and elsewhere comprise a middle film of plasticized polyvinyl butyral sandwiched between two facing glass plates. Such laminated glass has great strength and fragments do not fly about even if the glass is broken.
2. The Prior Art
A method of manufacturing the middle films used in such laminated process is known. This method feeds to an extrusion mold polyvinyl butyral that has been heated to melting while being mixed in a cylinder, and extrudes it from the extrusion mold to obtain a thin-film for use in the middle of the glass layers.
The middle film so obtained is colorless and transparent, and most of the light passes through this middle film. In order to restrict the transmission of light, attempts have been made to set up a band-shaped colored section along part of the width of the middle film.
For example, in the thermoplastic resin sheets used in automobile windshields, the amount of sunlight shining on the driver through the windshield can be reduced by creating a colored section with a color gradient at the top of the sheet.
The colored section is usually set up in the sheet by printing a band-shaped colored section on the surface of a film made of polyvinyl butyral. However, when the colored section is formed by applying ink, there is the disadvantage that the colored section peels away easily from the film surface. Moreover, after the polyvinyl butyral resin film has been formed, the colored section must be formed, and the manufacturing process is complex, and the equipment is unavoidably large scale.
In Japanese patent publication Hei-l-43606 (U.S. Pat. No. 4,316,868), a process is disclosed for manufacturing sheets with a certain width colored band encapsulated within the thermoplastic resin. In this process, the thermoplastic resin main flow goes into the manifold and is extruded through an extrusion passage and from a slit-shaped die extrusion mold, and colored thermoplastic resin is extruded from a probe orifice which is encapsulated in the main flow.
A 3-layer thermoplastic resin sheet in which colored thermoplastic resin is encapsulated in thermoplastic resin is obtained by using an extrusion mold that has a manifold into which thermoplastic resin is fed, has a probe located in said manifold that feeds colored resin into the manifold and has a slit-shaped die orifice that serves as the extrusion sheet outlet. The clear thermoplastic resin main flow goes into the manifold and is extruded through the slit-shaped die extrusion mold. The colored thermoplastic resin extruded from the wedge-shaped orifice provided in the probe is encapsulated in the main flow of resin.
This process has the following problems.
1. Because the colored resin is extruded parallel to the flow of the main resin, the boundary between the main resin and the colored resin encapsulated in the main resin is clear. In other words, the edges of the colored band are clearly visible. By giving the probe orifice a wedge shape, the boundary between the colored band and the main flow resin can be made a bit unclear, but there are limits to this technique.
2. Because the probe is offset to one side of the manifold, the flow of clear thermoplastic resin sent from the manifold through to the slit-shaped die orifice is nonuniform. The resin extrusion flow amount is nonuniform and the sheet widthwise thickness easily becomes nonuniform.
3. Because the probe is fastened to the mold on only one side, the probe position changes within the manifold and the colored belt shape and sheet thickness are unstable.